Natural gas from producing wells contains natural gas liquids (NGLs) that are commonly recovered. While some of the needed processing can be accomplished at or near the wellhead (field processing), the complete processing of natural gas takes place at gas processing plants, usually located in a natural gas producing region. In addition to processing done at the wellhead and at centralized processing plants, some final processing is also sometimes accomplished at Midstream NGLs Recovery Plants, also known as “straddle plants.” These plants are located on major pipeline systems. Although the natural gas that arrives at these straddle plants is already of pipeline quality, there still exists quantities of NGLs, which are recovered at these straddle plants.
The straddle plants essentially recover all the propane and a large fraction of the ethane available from the gas before distribution to consumers. To remove NGLs, there are three common processes; refrigeration, lean oil absorption, and cryogenic.
The cryogenic processes are generally more economical to operate and more environmentally friendly; current technology generally favors the use of cryogenic processes over refrigeration and oil absorption processes. The first-generation cryogenic plants were able to extract up to 70% of the ethane from the gas; modifications and improvements to these cryogenic processes over time have allowed for much higher ethane recoveries (>90%).